Field-effect transistors that use GaN, such as GaN high electron mobility transistors (GaN-HEMTs), are used as, for example, semiconductor devices that use a nitride semiconductor.
GaN has excellent material properties such as high withstand voltage, relatively high mobility in the case of using a two-dimensional electron gas (2DEG), and high saturation velocity and therefore GaN-HEMTs are being developed as power devices for power supply applications that enable high-power, high-efficiency, and high-voltage operation. In other words, GaN-HEMTs are being developed as power devices for power supply applications that enable high-power, high-efficiency, and high-voltage operation which is not easily achieved by Si laterally diffused metal oxide semiconductor (LDMOS) transistors and GaAs field-effect transistors (FETs).
In the use of GaN-HEMTs in power supply applications, the threshold is desirably increased so that a normally-off operation in which no electric current flows when a gate voltage is not applied is achieved. For example, there are a technique that separates 2DEG by forming a gate recess and a technique that cancels out 2DEG by forming a p-type semiconductor layer directly below a gate electrode.
In some semiconductor devices that use a nitride semiconductor, a gate insulating film is disposed on the entire surface of a nitride semiconductor layer, that is, on the entire surface including not only a region directly below the gate electrode but also a region above a channel region (access region), and furthermore a gate electrode is formed on the gate insulating film. In some other semiconductor devices, a gate insulating film composed of aluminum oxide is formed using O2 or O3 with strong oxidizing power as an oxidizing raw material for the purpose of decreasing the C concentration in a gate insulating film and suppressing the leak current.
In the case where a gate insulating film is formed on the entire surface of a nitride semiconductor layer in semiconductor devices that use a nitride semiconductor, when an oxidized region is not formed near an interface of the nitride semiconductor layer with the gate insulating film or when an oxidized region is formed near the interface of the nitride semiconductor layer with the gate insulating film but the oxygen concentration is low, it has been found that a high drain current is achieved but the threshold is decreased.
On the other hand, when an oxidized region is formed near the interface of the nitride semiconductor layer with the gate insulating film and the oxygen concentration is high, it has been found that a high threshold is achieved but the drain current is decreased.
The following is reference document:                [Document 1] Japanese Laid-open Patent Publication No. 2010-98141 and        [Document 2] Japanese Laid-open Patent Publication No. 2010-238838.        